Geophysical prospecting by application of electromagnetic surveys where a primary electromagnetic field is generated to induce a secondary electromagnetic field in an underground formation has become widespread. During the last decades, survey systems for performing such prospecting have become airborne. An airborne electromagnetic system may comprise a tow assembly connected to an aircraft, typically a helicopter, and further comprising a transmitter system for generating the primary electromagnetic field that induces the secondary electromagnetic field in the underground formation. The secondary electromagnetic field is detected by a receiver system.
Achieving a high magnetic moment has been a goal achieved by increasing the current or the area of the transmitter. Such straight forward improvements have previously resulted in larger frames or larger current sources, which have proven difficult to implement or operate in airborne systems.
Larger frames with larger transmitter coils and thus a larger area have proven much more difficult to handle operationally than foreseen.
Simply increasing the current has also proven difficult due to appearance of higher order electromagnetic effects becoming significant and thus imposing limitation on how early measurements can be made in transient electromagnetic (TEM) systems. This implies longer wait times, basically a “dead-time” before the current has decayed in a transmitter coil. One way to reduce the wait time is to use a conductor or wire where each core is isolated, such as a Litz Wire. It is an objective to able to measure at event earlier times or to provide an alternative way than using special conductors.
Likewise, increasing the number of turns in a transmitter coil has proven difficult due to higher voltages when switching on and off the current source from the transmitter coil. Higher voltages may cause devastating and dangerous voltage breakthroughs or simply reduce quality of the measurements.
U.S. Pat. No. 5,796,253 relates to time domain electromagnetic mapping technique for geophysical prospecting and discloses techniques where a sequence or succession of multiple half sinusoids transmitter pulses replaces a single half sinusoid waveform to achieve steeper decays of the or fall time of the current in the transmitter coil. The letter discloses circuits for generating sequences or pulse trains of such half sinusoids in uncoupled transmitter coils.
It is an objective of this disclosure to improve upon such limitations.